The failure of a building heating system during extreme cold, such as in a blizzard, can endanger the lives of the building occupants as well as lead to expensive property damage and expensive emergency repair bills. Although heating systems can fail for a variety of reasons, perhaps the least likely components of a heating system to fail are the furnace burner unit that generates the heat and the heat exchanger that uses the heat generated by the furnace burner unit to warm the air transmitted into the building rooms through the duct system. Heating systems typically fail when one of the blowers or one of the control elements fails. It would be a benefit, therefore, to have a heating system that included dual furnace control mechanisms each including its own blowers and control elements that could each independently control the operation of a shared furnace burner unit and heat exchanger so that when one of the furnace control mechanisms malfunctioned, the remaining furnace control mechanism could be placed into service restoring heat to the building preventing injury to the building occupants and property damage, such as broken pipes, to the building. With the heating system still in operation, non-emergency repairs could be made to the malfunctioning furnace control mechanism during normal business hours or, if required, when financial circumstances allowed. Because malfunctions can occur in an unoccupied building and many individuals could find it difficult to manually switch from one furnace control system to another, it would be a further benefit to have a controller mechanism selector mechanism for detecting an abnormal operating condition, malfunction, in the controlling furnace controller mechanism and automatically switching control to the other furnace controller mechanism which provides an indicator to alert the user that service is required for the malfunctioning furnace control mechanism.